Any 6-month period refers to any range of 6 months that can be identified. It does not mean half a calendar year. The day affected employees receive their notice of retrenchment is included in the 6-month period.
An employer notifies 4 employees of their retrenchment on 1 April. As long as the employer notifies at least 1 employee of their retrenchment on or before 30 September, the employer is required to notify MOM of the retrenchment exercises involving all 5 affected employees, including the 4 employees notified on 1 April.